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Molecular BiologyUniversity of Colorado

Accurate chromosome transmission to daughter cells is essential for cell proliferation and the maintenance of reproductive fitness, while chromosome segregation defects frequently lead to aneuploidy, the inheritance of abnormal chromosome numbers. Aneuploidy is strongly correlated with genetic disease and miscarriage and is also a hallmark of tumor cells. Thus, deciphering the mechanisms that ensure accurate chromosome duplication and segregation is vital to understanding not only normal cell division, but also abnormal cell division that leads to cancer and other genetic conditions.

Following DNA replication, chromosomes consist of pairs of replicated sister chromatids that are tethered together physically by cohesins, ring-shaped complexes whose subunits have been highly conserved through evolution. This association, or cohesion, of sister chromatids early in the cell cycle is critical for orchestrating sister chromatid segregation during mitosis because it promotes chromosome biorientation, the attachment of the kinetochores on paired sister chromatids to microtubules that emanate from opposite poles of the dividing cell, thereby ensuring that each daughter cell inherits one copy of each chromosome. Importantly, cohesins are also known to play critical roles in DNA damage repair and in the regulation of gene expression.

To better elucidate the molecular mechanisms of cohesion, we have comprehensively mapped the molecular locations of cohesin complexes across the budding yeast genome using chromatin immunoprecipitation followed by hybridization to high-density tiled microarrays. Our studies have revealed that cohesin distributions are highly reproducible under various growth conditions and in different strain backgrounds. Cohesins are particularly enriched in centromere- flanking or pericentromeric regions and in intergenic regions between convergently transcribed gene pairs (see figure). These non-random cohesin distributions strongly suggest the existence of spatial regulatory mechanism(s) that precisely position cohesins throughout the genome. Long-term goals of the research in my lab are to elucidate the molecular mechanisms that are involved in the targeting of cohesins to particular locations and to understand the biological significance of cohesin positioning.

A Sample of Ongoing Projects:

We have recently determined the distribution of Scc2/Scc4, a heterodimeric complex that mediates cohesin deposition on chromosomes. Our results indicate that the Scc2/Scc4 loader colocalizes with cohesins and that loader association at CARs (cohesin-associated regions) is independent of cohesin. We are currently investigating possible roles for epigenetic chromatin modifications and transcription in localizing the Scc2/Scc4 cohesin loader.

Budding yeast kinetochores mediate cohesin enrichment throughout centromere-flanking domains that are as much as 400-fold larger than core centromeric DNA. Our observations suggest that a nucleation and spreading mechanism is involved in the assembly of these domains. We are currently using a number of approaches to test the veracity of this model.

Pericentromeric cohesin enrichment is essential for the orderly segregation of homologous chromosomes in meiosis I, and in the segregation of sister chromatids in meiosis II. We are currently manipulating budding yeast kinetochores to alter the locations of cohesin domains to investigate additional roles for these domains in the regulation of key meiosis I events.

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